Integrins are a superfamily of cell adhesion receptors, which are transmembrane glycoproteins expressed on a variety of cells. These cell surface adhesion receptors include gpIIb/IIIa (the fibrinogen receptor) and αvβ3 (the vitronectin receptor). The fibrinogen receptor gpIIb/IIIa is expressed on the platelet surface, and mediates platelet aggregation and the formation of a hemostatic clot at the site of a bleeding wound. Philips, et al, Blood., 1988, 71, 831. The vitronectin receptor αvβ3 is expressed on a number of cells, including endothelial, smooth muscle, osteoclast, and tumor cells, and, thus, it has a variety of functions. The αvβ3 receptor expressed on the membrane of osteoclast cells mediates the adhesion of osteoclasts to the bone matrix, a key step in the bone resorption process. Ross, et al. J. Biol. Chem, 1987, 262, 7703. A disease characterized by excessive bone resorption is osteoporosis. The αvβ3 receptor expressed on human aortic smooth muscle cells mediates their migration into neointima, a process which can lead to restenosis after percutaneous coronary angioplasty. Brown, et al., Cardiovascular Res., 1994, 28, 1815. Additionally, Brooks, et al., Cell, 1994, 79, 1157 has shown that an αvβ3 antagonist is able to promote tumor regression by inducing apoptosis of angiogenic blood vessels. Thus, agents that block the vitronectin receptor would be useful in treating diseases, such as osteoporosis, restenosis and cancer.
The vitronectin receptor is now known to refer to three different integrins, designated αvβ1, αvβ3 and αvβ5. Horton, et al., Int. J. Exp. Pathol., 1990, 71, 741. αvβ1 binds fibronectin and vitronectin. αvβ3 binds a large variety of ligands, including fibrin, fibrinogen, laminin, thrombospondin, vitronectin, von Willebrand's factor, osteopontin and bone sialoprotein I. αvβ5 binds vitronectin. The vitronectin receptor αvβ5 has been shown to be involved in cell adhesion of a variety of cell types, including microvascular endothelial cells, (Davis, et al., J. Cell. Biol., 1993, 51, 206), and its role in angiogenesis has been confirmed. Brooks, et al., Science, 1994, 264, 569. This integrin is expressed on blood vessels in human wound granulation tissue, but not in normal skin.
The vitronectin receptor is known to bind to bone matrix proteins which contain the tri-peptide Arg-Gly-Asp (or RGD) motif. Thus, Horton, et al., Exp. Cell Res. 1991, 195, 368, disclose that RGD-containing peptides and an anti-vitronectin receptor antibody (23C6) inhibit dentine resorption and cell spreading by osteoclasts. In addition, Sato, et al., J. Cell Biol. 1990, 111, 1713 discloses that echistatin, a snake venom peptide which contains the RGD sequence, is a potent inhibitor of bone resorption in tissue culture, and inhibits attachment of osteoclasts to bone.
It has now been discovered that certain compounds are potent inhibitors of the αvβ3 and αvβ5 receptors. In particular, it has been discovered that such compounds are more potent inhibitors of the vitronectin receptor than the fibrinogen receptor.